Trenberth was one of several members of the IPCC to testify before the House Science and Technology Committee on Feb. 8 about the report's findings. He is the lead author of a chapter on observations on the earth's surface and in the atmosphere in the IPCC's most recent report. He is also head of the Climate Analysis Section at the National Center for Atmospheric Research in Boulder, Colo.

I'm a biologist by training, so I am not conversant in the details of climate models, but I often hear from people outside the workplace who want to discuss climate change with me.

Something I often hear from skeptics is that global warming is merely a correlation to increased atmospheric carbon dioxide. I try to explain that it's well supported, but that often is not convincing.

The problem, I think, it that non-scientists do not understand the difference between a mechanism-driven hypothesis that is tested via a correlation, and a random correlation. Laypeople often hear in the news media of random correlations, such as those between health outcomes and diet, that don't stand up in later studies. The public need to know that a mechanism-driven correlation is fundamentally different, in that it will make a variety of predictions that can be tested.

Best wishes!

Kevin Trenberth: Indeed, as scientists we are concerned with much more than correlations but we seek why those exist and what the processes are. The climate models include many but not all processes. And given that they can simulate the observed phenomenon, which is increasingly the case, then it provides more confidence in future projections. Kevin Trenberth

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Manassas, Va.: Even before I ever heard of "global warming", I always believed that the climate is extremely complex to understand and even harder to forecast. At certain points in time throughout millions of years, long before gases caused by the industrial age, temperatures have been much higher than now and much colder than 30 years ago (when there was a fear of "global cooling"). If there were no industrial gases then, what caused such climate changes? I never heard a response to that question.

Count me as a skeptic. It seems that this report's favorite argument is that there is no argument to its conclusions.

One last thing. I live in Northern Virginia, where a prediction of 2 inches of snow causes school closures, government delays, and almost universal panic. For days up to last night, the best that meteorology can buy predicted that it would start snowing at 4 AM in the area. Well, as of 8 AM, there is no snow (just some sleet) and a bunch of kids happy not going to school. My point is, if a simple forecast for the next few hours is missed, why should we believe in climate forecasts that will happen decades from now?

Kevin Trenberth: See some of the other responses about weather vs climate. Weather relates to the processes and instabilities in the atmosphere, but climate relates to systematic influences on the atmosphere. e.g. if the sun heats up, so will the climate. But there will still be weather and colder periods. Historically the main changes from glacials to interglacials have been caused by the changes in the Earth's orbit around the sun. The precession of the equinoxes occurs with about a 20,000 year period. The axis of the earth's rotation changes on 40,000 year periods. The shape of the orbit changes also. The last major ice age was 20,000 years ago. There are subsequent changes in atmospheric composition and ice on the surface that provide feedbacks and amplify these changes. Now we have changed the atmospheric composition, carbon dioxide has increased by 35% since 1750 (pre-industrial times) and half of that increase has occurred since 1970. It changes the greenhouse effect and alters the natural flow of energy through the system by about 1%. That is enough to produce global warming.

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Steaming in Maryland: While I appreciate the value of double- and triple-checking in science, really, this consensus on anthropogenic GW has been largely in place for between 5 and 10 years. But oil industry lobbyists and sociopathic conservatives ensured the public would be confused enough to doubt the science for a decade (Exxon alone spent $8 million doing this). What can our country learn from this ignominious period of our history about the value of listening to science when it comes to, well, scientific issues?

Kevin Trenberth: It is important to realize that science deals with facts and not beliefs. And it is easy to amplify uncertainties or misunderstandings such as a cold spell caused by weather. It has been disappointing that science has not been treated appropriately in recent years, and the well funded disinformation campaigns have not helped.

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Arlington, Va.: Thanks for doing this chat. I understand most of the predicted outcomes of global warming, but am curious about how two of them would interact. The theory is that if enough ice melts, the ocean currents that keep northern Europe warm would cease, thereby plunging them into somewhat of an ice age. Would that not then refreeze a great deal of water that had melted and counteract the original problem to some degree? Or would the freezing of northern Europe not be that severe? Thanks.

Kevin Trenberth: It is true that melting of ice in the north changes the density of the ocean and can affect ocean currents. It is projected that the Gulf Stream may slow down in future decades, bringing less heat to Europe. But because this change is caused by warming, the warming wins out and the net effect is still warming in the North Atlantic, but just a bit less than there would be without such changes. There is a lot of mythology about this that is wrong (such as in the movie "The day after tomorrow".)

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Washington, D.C.: What I don't understand is if global warming is such a big issue, then why doesn't Congress pass a law banning chlorofluorocarbons? It concerns that even when cases get to the Supreme court, such as Mass. v EPA, they're about such small things, like tailpipe emissions in future years. Can't we do something big? Thanks.

Kevin Trenberth: Chlorofluorocarbons contribute very slightly to global warming. They are a bigger culprit for the ozone hole and ozone depletion. They have been largely banned for that reason. The problem now is fossil fuel burning in cars and power plants and so on, which creates carbon dioxide.

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Annapolis, Md.: Hi-

First of all, I want to thank you and everyone that worked on this report. The document is excellent and also somewhat overwhelming.

I believe that the two most stunning statements in the document are these:

"The atmospheric concentration of carbon dioxide in 2005 exceeds by far the natural range over the last 650,000 years (180 to 300 ppm) as determined from ice cores.

The atmospheric concentration of methane in 2005 exceeds by far the natural range of the last 650,000 years (320 to 790 ppb) as determined from ice cores."

Wow. It is astounding to think that the current levels of these two gases in our atmosphere exceed the range seen in the last 650,000 years.

I would like to see how critics of this report could toss aside that evidence as meaningless.

Thank you again for your work on this report.

Kevin Trenberth: Thanks. Yes that kind of information should make anyone pause.

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Washington, D.C.: I am still skeptical about what man can do to stop global warming. We had two full ice ages before one plant/car was built. Every time you breathe, go to the bathroom, or clap the planet heats us. Plus, the rate the planet is heating up isn't abnormal or out of line with history. Short of leaving the planet and shutting down our economy, I don't see any steps we can take that will make any difference.

We should focus on clean air and water, stuff we actually can work on, rather than the new poster child of the enviro-left. Remember DDT? That used to be there poster child, we stopped shipping it to Africa, and millions of children died because of it.

I don't want the US economy to be the next victim of a mis-guided policy.

Kevin Trenberth: The first step is to identify if there is a problem, and we think there is. What to do about it is another matter and involves many factors and value systems. How much do we value the planet we leave to the future generations? My own experience tells me it is less what we do and more how we go about it and implement it. Politicians are a key in this. Obviously one should not change policy instantly or it will hurt the economy, but if implemented over years so that people can plan for it then it can help the economy by making us more energy efficient. For cars the time horizon is order 10 years. For coal fired power plants it is order 35 years.

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Vienna, Va.: Why are more and more people without a scientific background so willing to dismiss the analysis of those with specialized training? No matter what the issue, whether it be global warming, vaccines, etc, it seems like everyone feels like they are qualified to analyze the issue even though many do not have the training and expertise to understand the issue.

Kevin Trenberth: There is a lot that can be said that is based on science: facts. This is not about beliefs or religion. What to do about it may be a different story.

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Washington, D.C.: I have read that global warming's "dirty little secret" is that there is no fix, barring drastic and immediate switches to alternate energy sources, which don't seem ready for mass consumption yet anyway. In your brutally honest assessment, have we crossed the Rubicon here? Will lowering emissions have any positive effect?

Kevin Trenberth: Lower emissions will not do much for the next 30 years, but will make a huge difference thereafter say by 2100. This is because carbon dioxide has a long lifetime (>100 years) and so what we already have in the atmosphere will be with us for quite a while. And the oceans heat up only slowly. We should act to slow down the problem but we also must recognize the problem and plan for it. Slowing it down allows for more planning and adaptation, and less disruption.

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Detroit, Mich.: Although, human causes of global warming have been discussed in the press for a while now, what and when was the particular finding(s) that ultimately led the majority of the scientific community to conclude that this was real?

Kevin Trenberth: The IPCC was set up 1988 indicating concerns. The major assessments of the IPCC have occurred in 1990, 1995, 2001 and now 2007. In each one there has been a progression of confidence that global warming is happening and is due to humans, mainly starting in 1995. Mother nature has played a major role as warm years keep piling up and the patterns of change in winds and rain are now evident and can be reproduced in climate model simulations. This is new. Previously it was temperatures. There was no singular event in the science, but some singular events have occurred that have raised public awareness. These include especially the 2003 heat waves in Europe that killed over 30,000 people (and which we can demonstrate was related to global warming), and Katrina in the US. Hurricanes are natural, but if they are even a bit more intense, then the damage goes over a threshold and things break.

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Falls Church, Va.: Can you distinguish the difference between climate change and weather? Many skeptics of global warming point to the weather for evidence when this doesn't seem to be the same thing.

Kevin Trenberth: Yes this is often a source of confusion. There is enormous variety of weather that occurs naturally. I think of it as what goes on in the atmosphere. Climate is when there are systematic influences from outside the atmosphere, such as from the sun, the oceans, or land. El Nino is a climate phenomenon involving the tropical Pacific ocean and the global atmosphere. Heat from the ocean alters heating pattern sin the atmosphere that affects the jet stream and wave patterns in the atmospheric winds. So it affects weather patterns. So does global warming.

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Silver Spring, Md.: Fighting climate change is just one more justification for a "light" rail project that would take trees and homes in my neighborhood, just for the tracks. Zoning changes would finish the job -- this neighborhood of small houses and big trees would be gone. There is no such thing as "Smart Growth." Without stabilizing the human population, there is no stopping climate change. We talked about that in the 1970s. Why is population a taboo topic now?

Kevin Trenberth: I agree with you. Energy use is very much per capita related. In China the use per capita is 1/10th that of the US but they have so many more people. To bring up their standard of living to that of the US has consequences. Population should be a big part of this discussion.

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Richmond, Va.: Mr. Trenberth,

Ice core data going back hundreds of thousands of years from prior glacial periods and warming periods indicates that warming occurs first, then CO 2 levels increase. This seems to indicate that the increase in CO2 levels is an EFFECT of global warming, NOT the CAUSE of global warming. An easy physical explanation: CO2 has a lower solubility in warmer water than it does in colder water, so when the oceans warm, they release CO2.

Secondly, there is ample evidence of SOLAR FORCING for global warming. During the Maunder Minimum (1600's), there were almost no sunspots. During this period of time, the Earth cooled, and glaciers ADVANCED! Since then, sunspots have reappeared, and lo and behold, the Earth has warmed, and the glaciers have been retreating ever since. This process has been going on BEFORE INDUSTRIALIZATION!!!

To my point: There is evidence that the Earth is warming. There is evidence that CO2 levels are rising. But just because you can put the 2 together on a graph, DOES NOT make for a CAUSE and EFFECT relationship.

Sincerely,

Bret D Stauffer

Kevin Trenberth: You are absolutely right. In the case of the thousands of year fluctuations, the climate changes and carbon dioxide and methane amounts in the atmosphere respond through changes on land and in oceans. For instance, as things warm up, the carbon and plant material in soils decays much faster (as it does in summer but not in winter) and if wet it does so anaerobically producing methane, but if dry it does so aerobically producing carbon dioxide. However, we have measurements of the sun from space since 1979 and we know that the recent warming is not from the sun. Instead carbon dioxide has increased about 16% since 1970, and that does have a major effect.

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Peoria, Ill.: The earth has been warming and cooling like clockwork in fairly regular cycles for about 2.5 million years. After a warming trend, the earth plunges into an ice age, and then slowly recovers through a warming trend that keeps getting warmer until, you guessed it, another ice age. Man has not existed through ANY of these cycles except for the very teeny tiny end of the current cycle. And, even then, the period of man's industrial activity is even more miniscule.

For example, regarding sea levels, consider that it is well known that the Florida Keys are comprised of ossified coral. Coral grows underwater. 100,000 years ago the Keys were under water. Currently they are not under water, but they will be once again, as they have been, repeatedly, in geological history. It is known that, during previous periods, the peninsula of Florida was much wider than it is today, extending far into what is now the Gulf of Mexico. These types of natural events have been occurring repeatedly in cycles well before man existed. Man's emissions may enhance this particular cycle of nature, but I doubt if it is the CAUSE of this particular warming trend. The participation of industrialized man in this current warming cycle comprises about the last 1% of the warming period. To give a perspective, today's current warming period started many hundreds of thousands of years BEFORE the ancient Greeks.

It is terrible to imagine the pain of Venice, Italy, Mauritius, or the Florida Keys going under water due to global warming. However, I do not think that man is in charge of these events. Since the 1800s scientists have known that the earth's wobble on its axis, coupled with its eccentric (i.e.: not truly circular) orbit are the likely cause of these repeated cycles of warming and cooling. As mentioned above, man did not even exist during any of these cycles, except for about the last 1% of the current warming trend. (For reference see National Geographic, Sept 2004 issue. See the graphs.)

Man cannot control many natural events such as meteorites striking the earth, volcano eruptions, earthquakes, solar storms, and many other natural events. I am all for limiting emissions, and I do my part. However, I feel that the "Al Gore" mania, while probably sincere, underestimates the power of nature and exaggerates the power of man.

Kevin Trenberth: The fact that there are changes in the past in climate that are mainly related to changes in the earth's orbit about the sun, is all the more reason to believe that if there is an agent of change then the climate will respond. We can measure the changes in forcings of the climate system, and human induced changes are occurring at rates a hundred fold those in nature. So yes, natural variability is occurring, but it occurs mostly much slower than recent climate change. The exception is when a volcano puts gases and debris into the stratosphere, causing a cooling for a couple of years, fairly quickly.

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Washington, D.C.: How would you respond to a Senator Inhofe who characterized global warming as a hoax or a Rush Limbaugh who said it was bogus.

Kevin Trenberth: The answer has to be that there are demonstrable facts that are clear. The debate should be on what to do about it, not that global warming is real.

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Arlington, Va.: Fareed Zakaria had a great op-ed in yesterday's Post, basically saying that it's too late to do much about the global warming problem. Do you think it's too late? What can we practically do to make a significant impact without dramatically changing our Western, industrialized lifestyle?

Kevin Trenberth: It is late and we can not do much to alter the outcome for the next 30 years, but actions we take now make an increasing large difference from that point on. So what we start to do now makes a big difference by 2100. We should think hard about energy and whether we can sustain current practices? More likely we need to change for those practical reasons and develop renewable energy that is sustainable. We will have to do this sooner or later.

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Washington, D.C.: Is there something wrong if you're anti-global climate? I'm at EPA and instead of having people agree to disagree with me, I was told otherwise and lost the respect of many people.

Kevin Trenberth: You know climate change is not necessarily bad. But rapid climate change is. The ecosystems can not live with rapid change: they can evolve if it occurs slowly enough. You should think about how to best frame the issues, look into why you and other disagree and what you agree on, and focus on what are the real points of debate.

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Rockville, Md.: I don't deny warming, but I do want to expand the debate beyond the first question "Will it be warmer in the future?"

I understand why political debates can stick to one point to discuss, but this should be science. And we should be able to ask "Is warmer good or bad?" when there are clear scenarios in the future where warmer would help. As in the verge of an ice age.

When I see people trying to get to the next stage of the debate they usually are attacked as "deniers." What good does that do?

Kevin Trenberth: Yes if you move from New York to Miami you have different climate. No problem. But ecosystems can't do that. Warming may have some benefits: longer growing seasons, reduced heating bills. But it also has downsides: more severe storms, hurricanes, droughts, heat waves, and higher air conditioning costs. Let's debate what to do about it, and how to plan for it.

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Washington, D.C.: A major 1974 article that appeared in Time Magazine cited a number of international and well-respected scientists (including from the National Weather Service and American universities) for the proposition that global cooling could be occurring and that catastrophic changes to the global environment could result. Given this past history, should citizens be quick to accept this new scientific "consensus" that human-engineered global warming is occurring?

Kevin Trenberth: That was media hype. It was never accepted in the scientific community. Two things happened. 1) Scientists realized that major ice ages were related to the orbit of earth around the sun, and thus on a time scale of tens of thousands of years we were headed toward the next ice age. We still are. 2) After World War II there was a lot of industrialization and air pollution. Clean air acts were introduced and cleanup up the air in many places (London smog, Pittsburgh etc). The visible pollution causes cooling, and that was also a result that was accepted at that time. The two items got a bit mixed up in the press. But there was never any reports from the national Academy or elsewhere (like IPCC) to say that cooling was on the way. The situation now is quite different.

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Arlington, Va.: It's kind of funny that now that we finally have winter weather in the D.C. area. There was a lot of hubbub about global warming when it was 75 degrees in January. Do you think the fact that we now have "normal" weather shows that the warming is not really happening? What about the fact that we had some unseasonably cold days? Shouldn't that counter the unseasonably warm days? Or is the extremes that all of the country has been seeing the last few years all a part of the climate change problem? Thank you.

Kevin Trenberth: Yes. With global warming we still have weather, and we still have winter. We also have El Nino: there is one underway now and that is influencing the character of the winter. Other effects from the Indian Ocean affected the first part of winter and led to the warmth on east coast. Now the El Nino component is coming more to the fore (the main storm track farther south than usual). All these effects can dominate at a place or time. But the overall change is for one of warming. Ironically, more warming means the atmosphere hold more moisture and so we get greater snow falls as a consequence of warming. In upper NY state, the absence of ice on the lake helps too.

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Washington, D.C.: I have heard that the Earth periodically goes through periods of rapid cooling resulting in occasional ice ages. I know that this process takes thousands of years, but is the Earth due for another such period of icing in the not too distant future?

Kevin Trenberth: The latest assessment is that this is quite a long time off. Not sure off top of my head, but beyond 30,000 years. It relates to tilt of orbit, precession of equinoxes and shape of orbit.

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Rockville, Md.: Are there any non-atmospheric reasons or factors to change the temperature of the Earth? Would it be possible for the Earth to enter a region of thin dust or gases that would lower the Earth's temperature Is it possible for a band of gas to circle the solar system and have periodical changes to affect the climate? We have detected bands of matter beyond the outer planets. Do we pay attention to these factors?

Kevin Trenberth: Yes we should pay attention to any such factors. Having appropriate satellites in space to measure their effects and any changes on the sun is essential (but threatened by cut backs in budgets). Other factors relate to land use: changing plants, forests etc.

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Mt. Lebanon, Pa.: The "argument" that global heating - I'm an engineer, we don't have a word for a thermodynamic process called "warming" - may be due mainly to natural causes is a foolish and evil canard. The outcome of the turkey in the oven in not dependent upon whether he's being roasted by an electric oven or one fired by natural gas. The net result from either alternative is a crispy carcass. Perhaps all of us adults can dispense with that pointless debating dead end.

Aside from models - which are notoriously unreliable when abutted onto real time events - what physical evidence is clearly attributable to global atmospheric changes?

And are there local changes - again correlated with hard physical evidence - that move against the global trend lines? If so, what accounts for these local events?

Thanks much. Registered Engineer

Kevin Trenberth: Indeed the term "warming" is ambiguous. For some it means increases in temperature. In the case of global warming it should mean increases in heating, one consequence of which is increased temperature. The increase in carbon dioxide increases the greenhouse effect and produces a warming equivalent to about 1% of the natural flow of energy through the system. Some of that heat goes into drying: evaporation. Over the oceans, where it is always wet, this happens. If the ground is wet after a shower, and the sun comes out, the first thing that happens is the ground dries out and then the temperature rises. Water is the air conditioner of the planet, as it is for our bodies. If you run out of water you get heat stroke. These symptoms of warming are widespread now and physically consistent: rising temperatures, rising sea temperatures, melting glaciers, less snow cover, rising sea level (from expansion and added melt water), increased drought in subtropics, and increased rains at high latitudes (as air can hold more moisture), heavier rains and snows. These observed changes are now simulated in models: this is quite recent.

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Washington, D.C.: According to NASA's Jim Hansen, the last time Earth's temperature was five degrees warmer sea levels were 80 feet higher. Other studies suggest the polar ice sheets are melting much faster than previously thought. Do you believe that the IPCC report's estimated sea level rise of less than two feet to be low?

Kevin Trenberth: The IPCC states the values but also has caveats that ice sheet collapse could add to the values considerably. There is an inadequate basis for saying what that might be. Glaciers tend to surge and then stop. 130,000 years ago in the Eemian period, it was warmer and sea levels 4 to 6 m higher, and Greenland was much smaller. This could happen if we keep on the current path but probably on about a 1000 year timescale.

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Washington, D.C.: I have a comment and a question.

Comment: I wish we could get politics out of this whole process. I am sure you will have numerous Bush hatters blaming this on him, etc..

Question: I recently read about a study that attributed the rise in global temperatures to the rise in methane in the atmosphere. They said it?s a much better insulator then CO2, and they tracked the increase to increase in the world population of cattle. What is you take on this?

Kevin Trenberth: Methane is a substantial contributor to global warming, second to carbon dioxide. But it has a shorter life time of about 10 years. I am from New Zealand where methane emissions are the major problem along these lines. A lot of research is going on with feed for livestock to reduce methane emissions (which mainly come from the stomach through the mouth). Every bit helps.

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Washington, D.C.: The National Oceanic and Atmospheric Administration's Chris Landsea resigned a year ago from the IPCC and leveled charges that the IPCC, and you in particular, had a overly-politicized view of global warming trends. (Post, Hurricane Scientist Leaves U.N. Team, Jan. 23, 2005). Specifically, I believe that Landsea objected to the fact that some on the IPCC would "utilize the media to push an unsupported agenda that recent hurricane activity has been due to global warming." I assume that you disagree with Mr. Landsea. Do you believe that recent hurricane patterns have been negatively affected by global warming?

Kevin Trenberth: This is what the IPCC says in the Policy Makers Summary: "There is observational evidence for an increase of intense tropical cyclone activity in the North Atlantic since about 1970, correlated with increases of tropical sea surface temperatures. There are also suggestions of increased intense tropical cyclone activity in some other regions where concerns over data quality are greater. Multi-decadal variability and the quality of the tropical cyclone records prior to routine satellite observations in about 1970 complicate the detection of long-term trends in tropical cyclone activity. There is no clear trend in the annual numbers of tropical cyclones. " This was agreed to by the US Govt and crafted by the lead authors present (including me). Landsea's comments were not correct.

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Washington, D.C.: A lot of scientific research requires substantial sums of money to conduct. My guess is that money, depending upon the scientist or foundation at issue, comes from a variety of sources: government grants, corporate sponsorship, non-profit sponsorship, and universities, right? My question is: are there any uniform standards of conduct or other self-regulation imposed by the scientific community related to the acceptance of funding to ensure that scientists' results and analysis are not swayed by the source of their funding?

Kevin Trenberth: Most grants are peer reviewed. Publications are peer reviewed. Disclosure of funding sources is not as open as it could and should be. Reproducibility of results is a longstanding criterion for science.

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Alexandria, Va.: Even if global warming is not caused by man, why wouldn't we want to take steps to save our finite resources and live on a cleaner planet? Green technology exists and, if eased into use, will not cripple our economy. The US is the biggest polluter, by far, on the planet. Other countries have emission standards in place right now that far exceed our own, and their economies are doing great (see Japan, their fuel emission standards, and the efficient cars they drive in order to meet those standards). It's time for the US consumer to demand better; time for lawmakers to help make it happen.

Kevin Trenberth: Agree. Companies that have responded have seen huge savings by increasing energy efficiency. I predict that in 100 years our ancestors will say about us, "Look at all those petroleum resources, and what did they do with them they burned them!"

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Albany, N.Y.: Does the use of ethanol based fuels create less global warming than fossil fuels such as oil based fuels?

Also, why did our government place a tariff on the imported sugar cane used for ethanol production? It is my understanding that the yield of ethanol is much greater and therefore provides a more efficient conversion to energy than corn. Is this purely political or is there some scientific reason?

Kevin Trenberth: I believe it relates to lobbyists for sugar in this this country.

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Washington, D.C.: I saw an interesting show in the PBS series "NOVA" about global dimming--about how we're actually getting less sunshine, all over the world, than we used to because of a combination of increased cloud cover and pollutants. As a result, temperatures have actually been cooler than they might have been. Have the new reports taken that into account?

Kevin Trenberth: Yes in my chapter we have a special section on this. 1) global dimming was not in fact global: it did not occur over the oceans and observations were biased by urban areas, where pollution plays a role. 2) In other places, it relates to increases in cloudiness, this is true over much of US. That blocks the sun (dimming) but also provides a blanket (a greenhouse effect) so the net effect is not as big as advertised by some. Also more heat goes into evaporating moisture from the extra rains associated with the clouds, and thus temperatures have not warmed up as much as they would have otherwise.

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Wilmington, N.C.: "Now we have changed the atmospheric composition, carbon dioxide has increased by 35% since 1750 (pre-industrial times) and half of that increase has occurred since 1970. It changes the greenhouse effect and alters the natural flow of energy through the system by about 1%. That is enough to produce global warming."

I read a lot of news and this is the first I have seen this statistic. It is compelling. Would you mind terribly going on every radio and television show in the US and repeating it? Please.

Kevin Trenberth: Easier said than done.

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Washington, D.C.: You keep saying 2100....so what are going to be the unstoppable affects felt from now-till 2100?

Kevin Trenberth: Projected changes are much larger beyond 2100. Between now and then we expect to see more of what we already have signs of, but much bigger: more severe and long lasting droughts, heat waves and wild fires, especially in SW U.S. Risk of bigger and more intense hurricanes. Heavier rainfalls. Heavier dumps of snow episodically. More humidity. And a longer growing season!

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Potomac, Md.: There is no regulation on use of electricity all over the united states: no buildings, public or private, have motion sensor light control or time control switches, like you see in Europe, in hotels, apartments, complexes, etc. I see a enormous, huge waste of energy in this regard and would like to know if something could be enforced in this regard?

Kevin Trenberth: In Europe the per capita consumption of energy is 2.5 times less than in the United States. Surely we can do better? One reason is gasoline is much more expensive and so is electricity. The best force is probably the market place, something like a carbon tax, or other incentives for changes in behavior. The can be punative or rewards.

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Mt. Lebanon, Pa.: The earth precesses as you mentioned in the same way a top does when it spins. Have any models taken into account the changes in the earth's magnetic field - both strength and polarity - as it has swapped polarity in the earth's past? Internal earth currents from changes in the core induce this field; the field itself also affected by the solar wind. Ultimately, everything is connected - through physics.

Thanks much. Registered Engineer

Kevin Trenberth: Not to my knowledge. Believed to be a small effect.

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Hampton, Va.: Hi Dr. Trenberth:

How does the radiative forcing produced by anthropogenic CO2 compare to the orbital eccentricity radiative forcing that produces the ice ages?

Kevin Trenberth: Actually, if the Earth's orbit changes, the net radiative forcing may be very small or even zero. But what happens is the energy is redistributed. Easiest example is if the tilt is less: the same energy arrives but less at the poles, and so they get colder (tropics get hotter). But at poles the snow and ice build up and reflect solar radiation and that does result in net cooling.

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Washington, D.C.: Hi Kevin,

The Worldwatch Institute puts out a trends report each year called "Vital Signs." In their 2005 edition, they mention that air travel accounts for 2 percent of all human-caused CO2 emissions but nearly all NOx emissions found 8-15 km above the Earth's surface.

Does the new report address air travel's contribution to climate change? Have the facts above changed significantly in the new report?

Kevin Trenberth: Our report does not deal with specific sources. Two more reports are forthcoming: Working group II deals with vulnerability, impacts and adaptation. WG III deals with mitigiation and is more likely to address your question.

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Richmond, Va.: I've done some studying on my own, and from everything I've seen the CO2-global warming thing is questionable at best. Long term (100's of thousands of years) of ice core data says that CO2 levels have been rising and falling as the earth has been warming and cooling. It also says that the CO2 levels LAG BEHIND the temperature increase, which leads one to believe that the increase in CO2 levels is an EFFECT of global warming, not the CAUSE of global warming.

There has also been data that says that glaciers have been retreating since 1750. Prior to that, the Earth was in what was called a mini-ice age. There is ample evidence that this was caused by a drop in solar output, as evidenced by the LACK OF SUNSPOTS during the 1600's and early 1700's. What we are seeing today is a contrast from the earlier cool period when glaciers were ADVANCING!!!

Finally, geological data says that the Earth has been through 50 periods of glaciation of the past 5 million years, which means that we've been through 50 periods of GLOBAL WARMING over the past 5 million years. This is a quite normal phenomina.

Kevin Trenberth: You are partly right. Natural variations occur and volcanoes also play a role. It is likely that the little ice age was as much cause by volcanic aerosols blocking the sun as changes in the sun. But the sun has not changed noticeably since 1979 when we have excellent measurements of it. The atmospheric composition has. That has a major effect, but now it is humans.

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St. Leonard, Md.: I read your IPCC "Summary for Policy makers" and while I felt it was filled with great facts supporting the conclusion that global climate change is caused by human actions, it was lacking on its read-ability and big-picture context. The problem with global climate change has not been a lack of science, but a lack of public understanding. How can you expect policy makers to make a difference if they do not have an understanding of the context? Beating people over the head wih facts about how much the sea-level is going to rise doesn't seem like an effective way of making people change their lifestyles.

Kevin Trenberth: The document is not as user friendly as I would like. In our fill report we have a series of "frequently asked questions" which are all written in much friendlier english and with nice figures. Participating in this is another method of outreach.

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Mt. Lebanon, Pa.: And, of course, the changes you noted in the earth's physical structure are, in turn apart of other closed feedback loops which result in other changes to the earth's structure and climate.

I take it, the models you imply are at work in understanding these phenomena take into account these feedback loops. I'd love to see someone apply Mason's Gain Formula (for one) to simplifying these transfer functions.

Thanks much. Registered Engineer

Kevin Trenberth: Many processes are included but some are not. The carbon cycle is a case in point. As warming occurs and permafrost melts, soil carbon (bogs etc) will be released as either methane or carbon dioxide (depending on moisture presnet or not). That could amplify warming. Similarly methane hydrates (clathrates) in ocean sediments could be destabilized, etc etc.

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Cedar Falls, Iowa: What would your response be to someone who mentioned many of the strange theories of the past (not to say that global warming is strange) that were accepted as science? Isn't our knowledge of the climate and climatology still in its infancy? No field of science immediately starts out with everyone knowing all the answers. Many fields (medicine for example) take hundreds and hundreds of years to develop. To put it plainly, do you think you and your colleagues have a sufficient understanding of the climate?

Thank you.

Kevin Trenberth: We have a lot of understanding and we have built models that encapsulate that and which can be tested: on the annual cycle, on the past 100 years on past ice ages etc. And they are now much better and quite good. But they also need to get better. They are tools: useful tools, but still only tools. They are not the "answer" by themselves. They have to be used intelligently.

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Middletown, Md.: I see that someone from DC just asked a question that I was just about to ask about: Methane production. In your response, you noted that methane is a secondary contributor to warming behind CO2. However, other studies indicate that Methane is a much more potent greenhouse gas than CO2, and that focusing on CO2 reduction, while certainly helpful, ignores this major cause of warming. Also, as more cattle are bred for human consumption, this usually results in loss of vegetation which in turn can also cause CO2 levels to rise.

I lived in California during a serious drought in the 1980s. At that time, the politicians insisted that residential water use be restricted. However, they were uninterested in encouraging more efficient water use or limiting water consumption among the Central Valley farmers, who used 90% of the state's water. Having the non-farmers save 25% of their 10% of the water consumption really didn't do much, other than cause a lot of people a great inconvience.

Kevin Trenberth: Methane has shorter lifetime 10 vs >100 years) and what happens to methane? If burned it becomes carbon dioxide. As you note, one has toprovide the right incentives; often monetary

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Washington, D.C.: So, if we switched in 30 years (not unattainable, but we'd better start now) from coal and oil burning power plants to, say, nuclear power, and more people ride/use hybrids, public transportation, motorcycle/scooters, and bicycles, will this be sufficient for a while?

Kevin Trenberth: It will help: but it is a global problem. The US must become more engaged in developing global solutions and lead by example.

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Washington, D.C.: Just a comment: Thank you for answering questions today - even the tough, critical, or skeptical ones. Many kudos!

Kevin Trenberth: Thanks

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Katy, Tex.: You didn't answer the person's question on ethanol. Does converting to ethanol significantly/measurably change the CO2 output on a capita basis?

Kevin Trenberth: Ethanol is renewable: it comes from growing biofuels essentially. So while it may not change net output, it comes from carbon dioxide in air going into plants creating hydrocarbons and those are used for energy. Overall that is closer to neutral at least.

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Alexandria, Va.: All this discussion about global warming and the technologies that address this issue should not be looked at as a strain on the economy or way of life. Corporations, Congress and Consumers should look at the implementation of these technologies as an outstanding OPPORTUNITY. Why can't we keep our standard of living, but just change the way in which we achieve it? Some smart folks out there are going to make a fortune.

Kevin Trenberth: I agree. Other countries, especially Japan and Europe have the lead (look at Toyota). I think we could be amazed at what might eventuate given the right incentives.

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Kevin Trenberth: OK I am done, whew!

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